The centrosome is a conserved organelle that organizes the microtubule network, and nucleates the primary cilium, a nexus for cell signaling pathways. Centrosome and cilium dysfunction are associated with a variety of human diseases including cancer and ciliopathies. The most common centrosome abnormality observed in cancer cells is supernumerary centrosomes. Remarkably, the mechanism by which cancer cells acquire an abnormal number of centrosomes is not understood. Centrosome number in cells is maintained by a duplication and segregation mechanism linked to the cell cycle. Given the importance of segregation of centrosomes at mitosis, I propose that another potential mechanism leading to centrosome number abnormalities is missegregation on the spindle poles during cell division, analogous to the missegregation of chromosomes. The specific goals of this project are to understand how centrosomes become associated with the poles of the mitotic spindle, and the mechanisms that ensure their equal segregation, as well as defining the consequences of failure of centrosome segregation on mitotic progression and its downstream events. To identify proteins required for the attachment and segregation of centrosomes in mitosis, in aim 1 I will first use a targeted RNAi screen, focused on centrosome proteins, followed by a genome-wide RNAi screen in mammalian cells. After validating the specificity of the RNAi phenotype for the candidate proteins, in aim 2 I will characterize the most promising validated candidates for their role in centrosome segregation by assessing the RNAi phenotype, determining their localization and defining their interactions. This phenotypic analysis will allow me to build a mechanistic model for centrosome attachment and distribution. Little is known about how failure of centrosome attachment and distribution might affect mitotic progression and downstream events and in aim 3 I propose to assess the associated defects in chromosome segregation, spindle organization, cytokinesis and the subsequent cell cycle. The experiments in this proposal will provide valuable insight into the regulation of centrosome segregation during cell division and the mitotic defects associated with the failure of this regulation. The abnormalities in centrosome number contribute to the development of a variety of human diseases including cancer. A better understanding of the mechanism of centrosome attachment and distribution during cell division may someday lead to new diagnostics or treatments for these diseases.